This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
Smart/adaptive antenna beam forming, which allows for directional transmission and reception of wireless signals, is one of the key technologies in Time Division Duplex (TDD) mobile communications. For accurate directivity, beam forming requires individual radio paths to have the same phase and amplitude response. To satisfy this requirement, the so-called antenna calibration process must be performed, whereby the difference in phase and amplitude among radio paths can be compensated for.
Since transmission paths and reception paths within a radio unit are independent from each other, antenna calibration shall be further divided into transmission calibration and reception calibration. In the case of transmission calibration, each transmission path is calibrated relative to the other transmission paths. In the case of reception calibration, each reception path is calibrated relative to the other reception paths.
FIGS. 1 and 2 are schematic diagrams illustrating conventional approaches for transmission calibration and reception calibration, respectively. As shown in FIGS. 1 and 2, for both transmission calibration and reception calibration, the same hardware arrangement of a radio unit 100 connected with an antenna array 200 is used. Inside the antenna array, there is arranged a calibration network. The calibration network is consisted of multiple couplers 210-213, which enable signals received at the antenna array 200 from the radio unit 100 to be coupled back to the radio unit 100.
For transmission calibration, individually identifiable reference signals are initially generated by an antenna calibrator 110 within the radio unit 100. Then, the individually identifiable reference signals pass through individual transmission paths within the radio unit 100 and are coupled back to an antenna calibration (AC) port through the calibration network, respectively (as denoted by the dotted lines in FIG. 1). As shown in detail in FIG. 1, all the transmission paths have the same structure. Taking the second transmission path as an example, each of the transmission paths comprises a transmission signal processing section 121 followed by a power amplifier (PA) 131 and a bandpass filter 141. The transmission signal processing section 121 is consisted of a digital up-converter (DUC) 1211, a digital-analog converter (DAC) 1212, a frequency mixer 1213, an amplitude adjuster 1214 and an amplifier 1215.
Next, the individually identifiable reference signals pass through a reception signal processing section 170 and arrive at the antenna calibrator 110 (as denoted by the solid line in FIG. 1). As shown in detail in FIG. 1, the reception signal processing section 170 comprises a filter 1701, an amplifier 1702, a frequency mixer 1703, an amplitude adjuster 1704, an analog-digital converter (ADC) 1705 and a digital down-converter (DDC) 1706.
Finally, at the antenna calibrator 110, transmission calibration vectors used to compensate for the difference among transmission paths are computed by comparing the received signals with the reference signals.
For the reception calibration, a reference signal is initially generated by the antenna calibrator 110. Then, the reference signal passes through the first transmission signal processing section within the radio unit 100 and is directed to the AC port. Next, the reference signal is coupled back to the radio unit 100, passes through individual reception paths within the radio unit 100 and arrives at the antenna calibrator 110. As shown in detail in FIG. 2, all the reception paths have the same structure. Taking the first reception path as an example, each of the reception paths comprises a bandpass filter 140 followed by a low noise amplifier (LNA) 150 and the reception signal processing session 170.
Finally, at the antenna calibrator 110, transmission calibration vectors used to compensate for the difference among reception paths are computed by comparing the received signals with the reference signal.
As can be seen from the above, the conventional approaches for transmission calibration and reception calibration both rely on the calibration network outside the radio unit. Thus, from the perspective of the radio unit, the conventional transmission/reception calibration is called external antenna calibration.
Although antenna arrays with calibration network were widely adopted by veteran TDD operators, there are more and more newly-emerging TDD operators using antenna arrays without calibration network in their TDD communication networks.
In order for those new TDD operators to perform antenna calibration and thus to benefit from smart antenna beam forming, there has been proposed a radio unit with a build-in calibration network. As an example of the proposed radio unit, FIG. 3 illustrates a radio unit 300 comprising an antenna calibrator 310, transmission signal processing sections 320-323, power amplifiers 330-333, bandpass filters 340-343, couplers 350-353, reception signal processing sections 370-373, switches 360-362 and LNAs 410-413. The antenna calibrator 310, transmission signal processing sections 320-323, power amplifiers 330-332, bandpass filters 340-343, reception signal processing sections 370-373 and LNAs 410-413 of the radio unit 300 have their counterparts in the radio unit described above with respect to FIGS. 1 and 2, while the couplers 350-353 and the switches 360-362 are newly added elements which constitute the build-in calibration network of the radio unit 300.
With the architecture of the proposed radio unit 300, transmission/reception calibration can be performed even if the antenna array connected to the radio unit does not have a build-in calibration network. In this case, the antenna calibration relies on the calibration network inside the radio unit 300 and is therefore referred to as internal antenna calibration.
As shown in detail in FIG. 3, for calibrating the first transmission path within the radio unit 300, a reference signal generated by the antenna calibrator 310 passes through the transmission signal processing section 320 followed by the PA 330 and the bandpass filter 340, and is coupled back to the reception signal processing section 370 via the coupler 350, the switch 360 and the switch 361 (as denoted by the solid line in FIG. 3). Similarly, for calibrating each of the other transmission paths, a reference signal generated by the antenna calibrator 310 may pass through a corresponding one of the transmission signal processing sections 321-323 followed by the PAs 331-333 and the bandpass filters 341-343, and be coupled back to the reception signal processing section 370 via the coupler 350, the switch 360 and the switch 361 or 362. In this case, the switches 360-362 cooperate with each other to select only one transmission path at a time to be subject to internal transmission calibration. Accordingly, the internal transmission calibration must be performed with respect to different transmission paths in succession (i.e., at different times).
Likewise, for calibrating the first reception path within the radio unit 300, a reference signal generated by the antenna calibrator 310 is coupled back to the bandpass filter 340 via an attenuator 390, the switch 360, the switch 361 and the coupler 350, and passes through the bandpass filter 340 followed by the LNA 410 and the reception signal processing section 370 (as denoted by the dotted line in FIG. 3). Similarly, for calibrating each of the other reception paths, a reference signal generated by the antenna calibrator 310 may be coupled back to a corresponding one of the bandpass filters 341-343 via the attenuator, the switch 360, the switch 361 or 362 and a corresponding one of the couplers 351-353, and pass through the corresponding one of the bandpass filters 341-343 followed by the LNAs 411-413 and the reception signal processing sections 371-373. In this case, the switches 360-362 cooperate with each other to select only one reception path at a time to be subject to internal reception calibration. Accordingly, the internal reception calibration must be performed with respect to different reception paths at different times.
Being performed for individual transmission/reception paths at different times as set forth with respect to FIG. 3, the above-described internal transmission/reception calibration approach may suffer from inaccuracy due to the time-varying phase of the radio unit's local oscillator (LO).
Specifically, supposing the phase of the radio unit's LO changes over time as illustrated in FIG. 4 and the transmission calibration is performed with respect to two different transmission paths at different times T1 and T2, calibration inaccuracy for the different transmission paths would be caused by the difference between the phase of the radio unit's LO at the time T1 and the phase of the radio unit's LO at the time T2. Likewise, calibration inaccuracy for different reception paths would be caused by the difference among the radio unit's LO phases at different times.
Although the calibration inaccuracy caused by the time-varying phase of the radio unit's LO can be reduced by performing long-term average antenna calibration, the time and the radio resources required for antenna calibration would increase significantly.